This invention relates to a hydrometallurgical method of recovering zinc, copper and cadmium from the oxidic compounds of these metals and iron. The invention is particularly suitable for being used in connection with hydrometallurgical production of zinc.
The electrolytic zinc process uses as its principal starting material the oxidic product obtained from sulphidic zinc concentrate by roasting zinc calcine. It contains the zinc mostly as oxide (80 - 90 %), for a considerable part as ferrite (5 - 15 %), some as sulphate (2 - 5 %) and small amounts as unroasted sulphide, silicate and aluminate. The calcine, the zinc content of which is in general between 50 - 65 %, contains 3 - 12 % iron, about 1 % lead and silicon dioxide, some tenths per cent of copper, cadmium, manganese, magnesium, barium and aluminum. Calcine contains also minor amounts of silver (10 - 100 ppm) and gold (&lt; 1 ppm).
The iron in the calcine is almost completely in ferrite form, the principal product being zinc ferrite ZnFe.sub.2 O.sub.4. The ferrites have a decisive significance with regard to further treatment of calcine.
Almost up to the present time, selective leaching has been carried out with calcine in weak sulphuric acid solution so that the oxides dissolve while the ferrites stay mostly undissolved. The reason for that has been the difficulty in precipitating in an easily filterable form the big amounts of iron formed in leaching of ferrites. The leach residue formed as a result of such a leaching treatment consists thus mainly of ferrites containing also insoluble sulphates and other insoluble compounds such as silicates.
For further processing of this ferritic leach residue and for recovering the valuable materials contained in it, e.g. a hydrometallurgical method known by the name of jarosite process has been developed. It has been described in more detail in the Norwegian Letters Patent No. 108,047 and in the Article "Die Eisenfallung als Jarosit und ihre Anwendung in der Nassmetallurgie des Zinks" by G. Steintveit, Erzmetall 23 (1970) 532-539.
In the jarosite process, the ferritic leach residue is leached at a sufficiently high temperature (90.degree. - 95.degree.C) in the return electrolyte of zinc electrowinning (= spent electrolyte), whereby most of the ferrites dissolve. The solid material remaining in this leaching -- which contains the ferrites still left undissolved, difficultly soluble sulphates (PbSO.sub.4, CaSO.sub.4, BaSO.sub.4), most of the silicon (in the form of SiO.sub.2) and silver and gold -- can be separated from the solution and led to processes, where it is possible to carry out the separation and recovery of the valuable materials contained in it (Pb, Ag, Au). If the Pb-, Ag- and Au-contents are low, this separation is not necessary and the solution with its solid materials can be led -- as generally done at present -- directly to iron precipitation.
The solution formed in leaching of ferrites, the iron and sulphuric acid contents of which solution are most usually varying between 20 - 35 g/l and 40 - 80 g/l, respectively, has been led directly or after preneutralization to iron precipitation. Zinc calcine has been fed to this stage to neutralize the excess sulphuric acid and that liberated in precipitation reactions. In general, the pH-value has been endeavoured to be adjusted in the precipitation stage of the jarosite process to 1.2 - 1.4. When also sodium and ammonium salts have been fed to this stage, the iron is precipitated as sodium or ammonium jarosite (A[Fe.sub.3 (SO.sub.4).sub.2 (OH).sub.6 ]; A = Na, NH.sub.4). Here the characteristic of the ferric iron to form difficultly soluble basic sulphates in acid solutions in the presence of NH.sub.4.sup.+ -, Na.sup.+- and K.sup.+-ions under atmospheric conditions has been made use of (J. G. Fairchild, Amer. Min. 18 (1933) 543-547; G. P. Brophy, E. S. Scott, R. A. Snellgrove, Amer. Min. 47 (1962) 112-126; N. W. Sziszhin, Zap. Ws. Min. Obszcz. 79 (1950) 94-102; N. W. Sziszhin, E. A. Krogius, P. A. Lowowics, Zap. Ws. Min. Obszcz. 87 (1958) 682-686; Z. Harada, M. Goto, Kobutsugaku Zasshi 1 (1954) 344-355, Ref. Chem. Abstr. 51 (1957) 143 h).
The ferritic part of calcine used for neutralization does not dissolve in the precipitation stage, but remains in the iron precipitation. Therefore, acid wash of jarosite precipitate has been connected to the jarosite process (Norwegian Letters Patent No. 123,248 ). Sulphuric acid or spent electrolyte is sufficiently added to the thickened jarosite precipitate slurry, and the leaching conditions are maintained to correspond to those of the leaching stage of ferrites. Now most part of the ferrites in the precipitate dissolve while the jarosite remains undissolved.
Accordingly there can be four additional stages in the jarosite process in addition to neutral leaching: leaching of ferrites, neutralization of excess acid, precipitation of iron as jarosite and acid wash of the jarosite precipitate. Each one of these stages requires a separate adjustment and control system and a considerable number of reactors and thickeners (FIG. 1).
It is also possible to precipitate the iron in autoclave without neutralization either at a higher temperature (180.degree. - 220.degree.C) as a mixture of hematite and hydronium jarosite or in the presence of NH.sub.4.sup.+-, Na.sup.+- and K.sup.+-ions at a somewhat lower temperature ( 140.degree. - 180.degree.C) as ammonium-, sodium- and potassium-jarosite (U.S. Letters Pat. No. 3,493,365; Canadian Letters Patent No. 787,853).
In both of the methods described above, leaching of ferrites and precipitation of iron are carried out in different stages. In the method described first -- in its most common form -- this precipitation stage is combined with one preceding (preneutralization) and one subsequent stage (acid wash). In the latter method, precipitation of iron is carried out in autoclave, which both process and equipment technically is considerably more difficult to perform than atmospheric process stages.